Granular materials such as ceramics, granulated metals and cermets have been employed to form structural bodies having hollow portions. When such hollow portions are of straight or linear configurations, the structural bodies can be formed by molding techniques. For example, as seen in FIG. 1, a structural body of granular material 1 with a simple cylindrical hollow portion 2 can be easily formed by injection molding of a granular material combined with a binder of good flowability in which a core member of simple cylindrical configuration can be employed to form a hollow portion 2 and be removed after molding.
However, where hollow portions have configurations which are not simple and from which cores cannot be taken out after molding as, for example, shown in FIG. 3 in which a hollow portion 2' has an enlargement 2" at its central portion, this molding technique is not available any more.
As an alternative, the method as illustrated in FIG. 2 has been utilized in which a plaster mold comprising mating halves 3, 3 is provided with a centrally enlarged cavity and a slurry stock composed of granular materials and deflocculants at an appropriate concentration is poured into this cavity. The slurry stock is deprived of water by the cavity wall to form a thin pot-like body 4 having an enlarged portion 4'. Then, mating halves 3, 3 are opened to take out an undried body 4 which undergoes further drying and baking steps. This may be called the "slurry pouring method".
This method, however, has many disadvantages as enumerated below and is applicable only to limited areas:
(a) though a thin tubular or pot-like body 4 as shown in FIG. 2 can be obtained, other configurations like a structural body 10 as shown in FIG. 3 cannot be manufactured by this method; PA0 (b) the wall thickness of the body 4 attainable through absorption of water from the slurry stock touching the plaster cavity wall is physically restricted by the concentrations of slurry stocks used and the absorption ability of the plaster mold employed; PA0 (c) since the speed of thickening of the wall of the body 4 is extremely slow and the drying of the resultant body 4 is time-consuming, a large number of plaster molds are required for industrial and commercial processes; PA0 (d) because of difficult rheological problems to be solved on the powder/liquid systems and difficult control on the manufacturing steps affected by varying powdery stock materials, experience and expertness become important factors; PA0 (e) manufacturing steps and facilities are not suited to the automation, and resultant products are not fit for engineering use because of their coarse density as well as poor productivity.
Further, molded articles of granular materials, especially ceramics, have peculiar problems. Unbaked ceramic products are very unstable, heat-deformable, very frangible and of a nature to easily crack and lower their physical properties upon baking if foreign substances are present in the molding stock. Therefore, it has not been conceivable to utilize such core members of different material from that of molded structural bodies as used in metal casting, while ceramics and other granular structural bodies have advantageous characteristics in that, because of their substantial content of binder compositions, they may be efficiently manufactured by injection molding or transfer molding processes at low temperatures, which have been deemed not applicable to metal casting.